1. Field of the Invention
This invention relates to a magnetic head device employed in a magneto-optical disc system and a magnetic head driving circuit for driving and controlling the magnetic head device. More particularly, it relates to a magnetic head device employed for recording information signals on a magneto-optical disc and a magnetic head driving circuit for controlling the driving current driving the magnetic head device.
2. Description of Related Art
Proposed a disc recording apparatus is known which comprises an optical pickup device for radiating a light beam to a magneto-optical disc and for detecting the light returned from the magneto-optical disc A magnetic head device is arranged face the optical pickup device and applies external magnetic field to the magneto-optical disc. The magneto-optical disc is rotationally driven between the optical pickup device and the magnetic head device for recording information signals on the disc.
In the above-described disc the recording apparatus, recording of information signals on the magneto-optical disc is carried out by radiating a converged light beam from the optical pickup device on a signal recording layer of the magneto-optical disc for heating the recording layer. An external magnetic field is then applied by the magnetic head device on the heated portion of the signal recording layer for aligning the direction of magnetization of the heated portion with the direction of the applied external magnetic field.
Meanwhile, the magnetic head device employed in the abovedescribed disc recording apparatus comprises a magnetic field generating coil which is supplied with a driving current for generating a magnetic field and is controlled by a magnetic head driving circuit. The magnetic head driving circuit controls the driving current in accordance with information signals recorded on the magneto-optical disc and modulate the magnetic field generated by the magnetic field generating coil in accordance with the information signals.
The magnetic head device is controlled to follow the magneto-optical disc so that it is always spaced apart from the surface of the signal recording layer of the magneto-optical disc at a distance of approximately 100 .mu.m to 200 .mu.m. This movement is realized by providing a sheet of a uniform thickness on the surface of the signal recording layer and moving the magnetic head device on and in sliding contact with the sheet surface.
A magnetic head driving circuit as described in U.S. Pat. No. 4,937,802, has been proposed prior to the present invention.
The concrete example of the magnetic head driving circuit, proposed prior to the present invention, is hereinafter explained. A magnetic head driving circuit 114 comprises a plurality of field effect transistors (FETs) as switching elements, as shown in FIG. 1. The magnetic head driving circuit 114 is connected to a logic circuit 113 supplied with clock signals and recording signals which are the above-mentioned information signals to be recorded on the signal recording layer. The magnetic head driving circuit 114 comprises first to sixth FETs 115 to 120 having their gate terminals connected to the logic circuit 113 so as to be turned on and off by the logic circuit 113. The first and second FETs 115, 116 are P-channel FETs, while the third to sixth FETs 117 to 120 are N-channel FETs.
With the magnetic head driving circuit 114, a positive power source +V is connected to drain terminals of the first ad second FETs 115, 116. The first FET 115 has its source terminal connected to a source terminal of the third FET 117 and also to a drain terminal of the fifth FET 119 via a first diode 121. The second FET 116 has its source terminal connected to a source terminal of the fourth FET 118 and also connected to a drain terminal of the sixth FET 120 via a second diode 122. A magnetic field generating coil L is connected across the source terminal of the first FET 115 and the source terminal of the second FET 116.
The third and fourth FETs 117, 118 have their drain terminals connected to a negative power source -V, while the fifth and sixth FETs 119, 120 have their drain terminals connected to g round potential.
With the above-described magnetic head driving circuit 114, when the recording signal goes to a high (H) level, the first FET 115, which has been turned on when the recording signal is at a low "L" level, is turned off, while the second FET 116, which has been turned off when the recording signal is at a low "L" level, is turned on, as shown in a timing chart of FIG.2. At this time, the third FET 117 is turned on during a one-pulse period, based on the clock signals, after the recording signal has gone low. The fourth FET 118 is turned on in a pulsed fashion, based on the clock signals, during the period when the recording signal is at the H level, so as to follow up with the third FET 117.
With the above-described magnetic head driving circuit 114, when the recording signal goes to a low (L) level, the first FET 115, which has been turned off when the recording signal is at the high "H" level, is turned on, while the second FET 116, which has been turned on when the recording signal is at the high "H" level, is turned off. At this time, the fifth FET 119 is turned on during a one-pulse period, based on the clock signals, after the recording signal has gone low. The sixth FET 120 is turned on in a pulsed fashion, based on the clock signals, during the period when the recording signal is at the L level, so as to follow up with the fifth FET 119.
With the FETs 115 to 120 turned on and off in the controlled manner as described above, the voltage V.sub.A, applied across the magnetic field generating coil L, is changed from +V to -V after the recording signal has gone high and during the time when the third FET 117 is in a turned-on state. Since then, the voltage V.sub.A is pulsated in such manner that it is zero and at +V when the fourth FET 118 is in the turned-on state and in the turned-off state, respectively. When the recording signal goes low, the voltage V.sub.A is maintained at +V .
With the voltage V.sub.A across the magnetic field generating coil L controlled in the above-described manner, a current I.sub.H flowing through the magnetic field generating coil L is maintained at a substantially constant negative value as long as the recording signal is at an H level, and is maintained at a substantially constant positive value as long as the recording signal is at an L level.
Meanwhile, it is difficult to reduce the above-described magnetic head driving circuit in size because of the large number of components making up the driving circuit. Furthermore, it is necessary to employ a complicated control circuit for controlling the magnetic head driving circuit because of the large number of elements turned on and off in controlled manner.
The complicated magnetic head driving circuit is employed in the prior art driving circuit because it has been necessary to increase the magnetic field generated by the magnetic field generating coil L to a sufficiently high level. That is, in view of the characteristics of the signal recording layer of the magneto-optical disc, it has been necessary with the prior art driving circuit to apply a sufficiently large magnetic field even if the magnetic head device is spaced apart from the signal recording layer by a distance on the order of 400 .mu.m.